On Wed, Oct 06, 2021 at 10:11:29AM IST, Andrii Nakryiko wrote:

On Tue, Oct 5, 2021 at 5:29 PM Kumar Kartikeya Dwivedi [off-list ref] wrote:

quoted

Since the code assumes in various places that BTF fd for modules is
never 0, if we end up getting fd as 0, obtain a new fd > 0. Even though
fd 0 being free for allocation is usually an application error, it is
still possible that we end up getting fd 0 if the application explicitly
closes its stdin. Deal with this by getting a new fd using dup and
closing fd 0.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 tools/lib/bpf/libbpf.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index d286dec73b5f..3e5e460fe63e 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -4975,6 +4975,20 @@ static int load_module_btfs(struct bpf_object *obj)
                        pr_warn("failed to get BTF object #%d FD: %d\n", id, err);
                        return err;
                }
+               /* Make sure module BTF fd is never 0, as kernel depends on it
+                * being > 0 to distinguish between vmlinux and module BTFs,
+                * e.g. for BPF_PSEUDO_BTF_ID ld_imm64 insns (ksyms).
+                */
+               if (!fd) {
+                       fd = dup(0);

This is not the only place where we make assumptions that fd > 0 but
technically can get fd == 0. Instead of doing such a check in every
such place, would it be possible to open (cheaply) some FD (/dev/null
or whatever, don't know what's the best file to open), if we detect
that FD == 0 is not allocated? Can we detect that fd 0 is not
allocated?

We can, e.g. using access("/proc/self/fd/0", F_OK), but I think just calling
open unconditonally and doing if (ret > 0) close(ret) is better. Also, do I
leave it lingering, or should I close(0) if we created it on destroy?

Doing something like that in bpf_object__open() or bpf_object__load()
would make everything much simpler and we'll have a guarantee that fd
== 0 is not going to be allocated (unless someone accidentally or not
accidentally does close(0), but that's entirely different story).

quoted

+                       if (fd < 0) {
+                               err = -errno;
+                               pr_warn("failed to dup BTF object #%d FD 0 to FD > 0: %d\n", id, err);
+                               close(0);
+                               return err;
+                       }
+                       close(0);
+               }

                len = sizeof(info);
                memset(&info, 0, sizeof(info));
--
2.33.0

--
Kartikeya